A graph consists of a list of vertices (also known as nodes) and a list of connected edges.
The algorithms used in Graph'O->Mat try to display a graph in that manner to avoid crossings of the edges. The length of the edges has the highest priority in some algorithms. That avoids to have quite confusing graphs. Then the algorithms try to avoid crossings of the edges. If it is able to draw a graph without crossing (in two dimensions), then the algorithms manage to do so if there are not too many vertices (not more than approximately ten) and of course not too many edges.